The invention relates to a coherent radar comprising a magnetron, a modulator for driving the magnetron for the generation and transmission of HF-pulses, a stable local oscillator for producing an intermediate frequency signal of a frequency corresponding to the difference between the local oscillator frequency and the frequency of the transmitted pulses by mixing the local oscillator frequency with echo pulses caused by the transmitted pulses and an oscillator operating at intermediate frequency, the output signal of which is led, together with the intermediate frequency signal obtained by the mixing, to a phase sensitive detector for detecting the echo pulses and producing a signal, called bipolar video, inter alia containing information about the mutual phase position between the transmitted and received HF-signals. After a filtering operation the bipolar video signal can be used, both for unambiguous distances and for ambiguous distances, to separate targets having different radial speeds relative to the radar station, for example by suppressing echoes from fixed objects and only indicating movable targets.
When a magnetron is used as a HF-source in a coherent radar of this kind it is important that it can be made to deliver oscillations having a fixed phase relationship to the mixing product of the signal from the intermediate frequency oscillator and the signal from the stable local oscillator from pulse to pulse.
In a previously proposed solution of this problem, described in the Swedish patent SE 8400140-3, corresponding to U.S. Pat. No. 4,682,178 the modulator and the intermediate frequency oscillator are mutually time controlled in such manner that the front flank of the modulator pulse and thereby the magnetron pulse always appears in a predetermined phase position of the output signal of the intermediate frequency oscillator, and furthermore a phase stable signal, suitably derived from the already present stable local oscillator, is fed to the tuning cavities of the magnetron in an interval before triggering of the magnetron, so called priming. Such priming involves that a given quantity of oscillating energy is present in the cavities of the magnetron when the magnetron is triggered and starts to build-up own oscillations. This results in that the phase of these oscillations will be locked to the priming signal, which is derived from a continuously operating stable oscillator. Hereby phase continuity of the magnetron oscillations from pulse to pulse will be obtained.
A drawback for the solution according to the Swedish patent 8400140-3 is that the magnetron oscillations are not phase locked during the whole magnetron pulse but the phase can be subject to drift during the pulse itself. This can result in poor MTI-performance, in particular for extended magnetron pulses, while the MTI-performance for short pulses can be relatively good.